1. Field of the Invention
The present invention relates, in general, to a support for a dual-cooled segmented fuel rod and, more particularly, to a perforated plate support for a dual-cooled segmented fuel rod, capable of stably supporting the fuel rod to the end of its cycle even if an gap between the fuel rods becomes narrow due to application of a dual-cooled fuel rod, and of reducing vibration induced by flows of the inside and outside of the dual-cooled fuel rod for obtaining high burnup and power.
2. Description of the Related Art
A nuclear fuel assembly is charged in the core of a pressurized water reactor. This nuclear fuel assembly is composed of a plurality of fuel rods, in each of which a cylindrical uranium sintered compact (or a cylindrical uranium pellet) is inserted.
The fuel rods can be divided into two types, cylindrical and annular, according to shape. The annular fuel rods are called dual-cooled fuel rods.
In comparison with the pellet of the cylindrical fuel rod, the pellet of the annular fuel rod has a low internal temperature due to a thinner thickness and a wider heat transfer area, and thus a relatively higher safety margin.
FIG. 1 is a schematic front view illustrating a conventional cylindrical nuclear fuel assembly. Referring to FIG. 1, the nuclear fuel assembly 100 includes fuel rods 101, spacer grids 105, guide thimbles 103, an upper end fitting 107 and a lower end fitting 106.
Each fuel rod 101 has a structure in which a uranium sintered compact or a uranium pellet (not shown) generating high-temperature heat through nuclear fission is enclosed by a zirconium alloy cladding tube.
Each fuel rod 101 has upper and lower end plugs 108 and 109 coupled to lower and upper portions thereof so as to prevent inert gas filled between the cladding tubes thereof from leaking out.
Meanwhile, the structure of the fuel rod 101 has a length considerably long compared to the diameter thereof When this structure having a great slenderness ratio is subjected to coolant flow, the fuel rod 101 causes flow-induced vibrations due to the flow of the coolant. Thus, in order to reduce these flow-induced vibrations, the structure called a spacer grid 105 is installed in a predetermined section with respect to the entire length of the fuel rods 101 so as to support the fuel rods 101, thereby preventing the fuel rods 101 from being vibrated by the flow of the coolant.
However, in the case of the dual-cooled fuel rod designed to charge nuclear fuel into an annular space defined by a dual tube of inner and outer tubes, the spacer grid taking charge of an important function of inhibiting the vibration of the fuel rods caused by the flow of the coolant has no choice but to support only the outer tube of each fuel rod due to its structure. Due to the limitation of this supporting structure, in the case of the inner tube having the slenderness ratio of about 400 or more, only opposite ends of each fuel rod are supported by the upper and lower end plugs.
Of course, in the case of the dual-cooled fuel rod, a uranium dioxide (UO2) pellet exists between the inner and outer tubes. Thus, the vibration of the inner tube is expected to be inhibited to a certain extent. However, in cases of a fuel rod having an slenderness ratio of about 400 or more, it is easily surmised that a vibration amplitude of the inner tube is remarkably great, as compared to the outer tube having numerous support points formed in an axial direction of the fuel rod by the spacer grid.
The flow of the coolant in the internal coolant channel defined by the inner tube of the dual-cooled fuel rod can be interpreted as a problem of vibration of the tube in which fluid flows. Thus, if the vibration of this inner tube is not properly controlled, it is difficult to avoid damage to the fuel rod due to such vibration.
Further, there are additional considerations to consider in the case of the application of the dual-cooled fuel rod 10. In order to be structurally compatible with the core of the existing pressurized water reactor illustrated in FIG. 2, structural limitations that cannot change positions of core structural components, for instance guide thimbles 140, in the nuclear fuel assembly 100 must be accepted, and the outer diameter of the fuel rod yet must still be increased. As such, an gap between the dual-cooled fuel rod and the spacer grid has no alternative but to be considerably narrowed compared to an existing gap. For example, as in FIG. 3, if the nuclear fuel assembly is configured so that the positions of the guide thimbles 140 are maintained without change according to a design of the nuclear fuel assembly forming a 12×12 array, the gap between the dual-cooled fuel rod 10 and the unit spacer grid is reduced from 1.45 mm, which is the existing gap, to about 0.39 mm. Thus, due to the narrow gap between the dual-cooled fuel rod 10 and the unit spacer grid, technology that forms a fuel rod supporting structure on a surface of the unit spacer grid in order to support the fuel rod has until now been difficult to apply to the dual-cooled fuel rod.